A bit unrealistic in that it's like:
"OMG, some mysterious stranger is wrecking up our store!"
*Man walks by with a blowtorch in hand and an acetylene tank strapped to his back*
"You think it was that guy?"
"Nah, he had a kind face..."
(What I am saying is that it would make it more of a challenge if you had to avoid contact with employees after whenever it is that they are first alerted by the customers.) It would also give it more of a difficulty curve if the rival store deployed steadily-heightening countermeasures. As it stands, I was able to wreck stuff up enough that I got the blowtorch on like level 6 or 7 and it was smooth sailing until like level 10 (and especially level 10, lol...) Then the balance sort of swung the other way for the last two levels and you had no more upgrades left and levels that are getting more difficult.
Now that all the criticism is out of the way, I must say that this is a hilarious concept and a whole lot of fun to play. It has the same appeal as the "Just Cause" series, and for the same reasons (the main one being that you get to cause a bunch of nonsensical mayhem.) I think it could be better if you fixed the balance issues and added a little more strategy, though (rival store hitting you back, upgrading your store's security or coming up with sneakier ways of sabotaging the rival store are just a few ideas.)